1. Field of the Invention
The present invention relates to a method of simultaneously superfinishing a left and a right flank of a Gothic-arch groove of a ball thread, a linear movement guide bearing, a ball bearing, or the like in which balls roll.
In this respect, the Gothic-arch groove means a groove whose cross section orthogonal to the groove has a shape in which the centers of identical circular arcs of right and left flanks are offset.
2. Description of the Prior Art
In the prior art, generally, the Gothic-arch groove, which is a groove for balls to roll therein, of a ball screw, a linear movement guide bearing, a ball bearing, or the like has only been finished by grinding and has not been worked by superfinishing.
Furthermore, it has been known to work the superfinishing only on a surface portion of the Gothic-arch groove in the vicinity of a contact point of the ball used therein without working the whole surface of the groove.
In this prior art method, as shown in FIG. 13, in a Gothic-arch groove formed by offsetting a center O.sub.R of a circular arc having a radius R of a right flank 1 from a center OL of a circular arc having the same radius R of a left flank 2 in a horizontal direction by a distance a, an intersection point O.sub.B of straight lines respectively drawn from the centers O.sub.R and O.sub.L at an angle of 45 degrees with respect to the horizontal direction is the center of a ball 3 used therein, and the intersections P.sub.R and P.sub.L of the straight lines and the right flank 1 and the left flank 2, respectively, are the contact points of the ball 3 with the right flank 1 and the left flank 2. A superfinishing stone 4 having a radius larger than the radius of the ball 3 and smaller than the radius R of the groove is oscillated about an oscillation axis passing through the center O.sub.B of the used ball 3 and perpendicular to the drawing to thereby work the superfinishing on the vicinity to the fulcrums P.sub.R and P.sub.L of the Gothic-arch groove With respect to the used ball 3.
In this case, the reason for the use of the superfinishing stone 4 having the radius of its cross section larger than the radius of the ball 3 and smaller than the radius of the right and left flanks 1 and 2 is that fitness during the progress of the superfinishing is taken into consideration.
However, in the prior art superfinishing method of the Gothic-arch groove, the superfinishing is worked primarily in the vicinity of the contact points of the ball with respect to the groove, and the whole surface of the Gothic-arch groove is not worked uniformly. As a result, the radius size of the groove and the amount of offset which have been formed in a previous process (cutting work) are changed in the superfinishing, and the shape of the curved groove is degraded and the roughness of the surface becomes nonuniform. Thus, a problem is involved in that the evaluation of a value of the radius size of the groove after the superfinishing work becomes difficult.